This invention relates to a system for sensing chemicals in order to identify chemicals present in a test material, and in order to determine the concentration of the identified chemicals in the test material.
The invention particularly relates to a system in which chemicals are sensed using the optical properties of a specialised architecture of wave guides providing either an individual chemical or physical sensor, or alternatively a number of such sensors which may be included in an array based device.
Further, the present invention relates to a system for the abstraction of data and control of interferometric chemical sensor devices.
Measuring chemical species using conventional chemical sensor technologies does not provide the level of sensitivity and robustness required and does not provide a required performance for any important applications.
Current chemical sensors incorporating transducers do not offer the sensitivity displayed, for example, by the human nose.
Known optical chemical sensors, work on the principles of interferometry.
It is known to make use of optical waveguides in which the evanescent field (that is, the field which extends outside the guiding region) is used to sense discrete changes in optical properties. This known method relies on xe2x80x9cleakagexe2x80x9d of optical signals from a conventional wave guide structure into a sensing layer which is typically formed from an absorbent polymer.
The evanescent component of the optical signal being guided by the wave guide is typically small leading to limited interrogation of the sensing layer.
Another disadvantage of known waveguides used as chemical sensors is that the wave guides are very sensitive to changes in temperature. As a consequence, the sensitivity of these known devices is significantly attenuated.
According to a first aspect of the present invention there is provided a sensor system having a laminate structure comprising means for providing electromagnetic radiation; two or more wave guides; propagating means for simultaneously propagating electromagnetic radiation into the two or more wave guides, each of the wave guides comprising a planar wave guide layer formed in a substrate.
In known sensors incorporating optical waveguides, a sensing layer is modified by the chemical to be tested, and evanescent excitation radiation which interrogates the sensing layer is a small fraction of the total radiation contained in the waveguide layer. As a consequence, the sensitivity of such known devices is significantly attenuated.
In contrast, the present invention incorporates a sensor in which the sensing layer is used as the wave guide for the electro-magnetic radiation, thus ensuring the majority of excitation radiation interrogates the material.
The system of the present invention therefore benefits from significant improvements in efficiency when compared to known sensors using evanescent wave strategies.
The present invention further comprises a reference wave guide in addition to a sensing wave guide, which reference wave guide is provided internally within the sensor.
The leads to a simplified structure.
The present invention provides a chemical sensor with improved reliability. The range and applicability of a chemical sensor according to the present invention is thus greatly enhanced.
The present invention provides a chemical sensor system with a novel architecture of wave guide structure which leads to a robust device with significantly enhanced signal to noise ratios (sensitivity) and discrimination.
In addition, the present invention may be used to make physical measurements relating to pressure, position or vibration or test environment or material.
The device of the present invention is suitable for detecting changes in an ambient chemical environment without being excessively stressed by other environmental factors such as temperature.
The device according to the present invention comprises an optical quantum well sensor (OQWS) which comprises two or more wave guides one of which wave guides is modified in response to chemical species present in the environment.
Preferably, the means for providing electro-magnetic radiation provides electro-magnetic radiation having a wavelength falling within the optical range.
However, it is to be understood that the invention is applicable to devices adapted for use with electro-magnetic radiation of any wavelength, for example, infra-red and ultra-violet.
Advantageously, the means for providing electro-magnetic radiation is adapted to provide plane polarised electro-magnetic radiation.
Conveniently, the system further comprises propagating means for substantially simultaneously propagating electro-magnetic radiation into the two or more wave guides.
The device may further comprise excitation means for exciting substantially simultaneously a dual or multiple symmetric wave guide system.
Preferably, the device further comprises measuring means for measuring phase changes in the electro-magnetic radiation in each of the two or more wave guides.
According to a second aspect of the present invention there is provided an apparatus comprising a plurality of sensor systems according to the first aspect of the invention, each of which devices is arranged into an array.
The sensing wave guide may be formed from silicon. Alternatively, the sensing wave guide is formed from polymeric material such as polymethyl methacrylate.
The reference wave guide may be formed from silicon oxynitride. Alternatively, the reference wave guide is formed from a polymeric material such as poly-4-vinyl pyridine P4VP. Conveniently, electrodes positioned in contact with a surface of the sensing layer enables simultaneous capacitance and optical changes to be measured as a consequence of absorption on a single device.
Conveniently, the device comprises an integrated optical device.
Conveniently, the device comprises a plurality of optical quantum well sensors.
Advantageously, the device comprises a plurality of sensing wave guide layers each of which is laid down in a laminar fashion.
Using electro-magnetic radiation of different frequencies varies the contributions of the various laminations and may further enhance the utility of the device. Preferably, both excitation modes may be used to interrogate the sensor. In other words, both the TE (transverse electric) and the TM (transverse magnetic) modes are used.
In known devices the TE mode only would be used. However, the inventors have realised that by using the TM mode as well further information may be provided.
The device according to the present invention may be a passive device, or alternatively through use of appropriate materials it may be an active optical device.
Substrates such as quartz may be used to provide an active optical device. In such a situation active feedback mechanisms may be used to compensate for temperature changes thus negating the need for a photo detector array.
The use of a plurality of wave guides leads to a highly optimised wave guide device and allows simple optical arrangements to be used.
The device according to the invention thus has fast response characteristics. The path length of each of the plurality of wave guides is long which provides greater sensitivity.
In a preferred embodiment, the wave guides are built onto a substrate through lamination processes. Such processes are highly repeatable and lead to accurate manufacture.
Due to the laminate structure of the device, the sensing wave guide and the reference wave guide are in close proximity to one another. This minimises the effects of temperature and other environmental factors.
A mixed mode device may be utilised where fundamental physical concepts such as birefringence are also used to provide additional information concerning the test chemicals.
An interference pattern is obtained when the electro-magnetic radiation from both wave guide layers is coupled into free space and is recorded.
The interference pattern is used to determine the relative phase change which has occurred in the sensing wave guide with respect to the reference wave guide during the passage of electro-magnetic radiation through the structure. These are manifested as movement in the fringes and may be measured either using a single detector which measures changes in the electromagnetic radiation intensity, or an array of such devices which monitors the precise changes to the interference fringes. From the data obtained information about changes in the chemical nature of the sensing wave guide layer may be made.
The relative phase changes of the two modes are used to identify and quantify the nature of the optical changes taking place in the sensing layer. The relative phase changes of the two modes may also be used to identify such changes taking place in subsequent layers when more compact structures are employed.
The electrodes which may be laid down on the sensing layer may take the form of either parallel plates laid along side the wave guide structure or as an interdigitated or meander system laid down on the top and bottom surfaces of the sensing wave guide layer.
In the case of a meander system, the metal forming the electrode is responsible for scattering excessive amounts of light and as such the capacitance is measured on an adjacent structure which is not utilised for optical measurement. In both cases, the electrodes enable changes in capacitance of the absorbent layer to be measured.
Conveniently, measurement of capacitance and refractive mode index of the two modes yields further information on changes occurring in the absorbent layer.
The device according to the present invention may be used for making remote chemical sensor measurements and involves optical sensors of the type described hereinabove. Other technologies could also be used such as piezo-electric sensors or hot ceramic oxide devices, and condensed phase materials.
The invention can be used in a single mode of operation or in a bi-modal fashion providing additional data.
According to a third aspect of the present invention there is provided the use of a device according to the first aspect of the invention for determining chemical species.
Advantageously, there is provided a system for accurately determining chemical species, the system comprising a device according to the first aspect of the present invention further comprising:
means for providing a purge stream; and
means for collecting and analysing the data.
Calibration is a standard practice with laboratory based analytical equipment. Long term chemical sensing installations pose a particular problem in that detailed in situ calibrations are often inconvenient or not possible at all. As a result, the reliable performance of such devices is often impaired.
The chemical sensor system identifies chemicals of interest and is then periodically purged via a negating system using an appropriate valve system to channel the required gas streams.
The characteristics of removal of materials from the system are used to identify the analytes of interest. In using the approach, long, term drift of the sensors is compensated for, the relative response being measured rather than the absolute. In this manner, much greater sensitivity can be obtained with as long term drift of the system does not have to be accounted for when determining the limits of detection.
Typical improvements are of the region of an order of magnitude.
By measuring de-absorption aspects of the system some measure of the drift of the system can also be obtained.
The system can be run in low power mode if the power budget is limited.